1. Field of the Invention
The present invention relates in general to a support structure for a rotation shaft for automotive power steering, and relates in particular to a simple support structure which provides a firm engagement with the shaft to prevent an axial displacement of the shaft.
2. Technical Background
Power steering has become increasingly common for automobiles providing assistance to the steering effort required to turn the wheels of an automobile. The power steering mechanisms can be driven with either hydraulic or electrical device. Electrical motor driven systems are disclosed, for example, in a Japanese Utility Application, First Publication, H-4-54459 or a Japanese Utility Application, Second Publication, H5-44576.
FIG. 4 shows an example of the structural details of a drive unit in a rack and pinion steering system. The rack and pinion type driving system shown in FIG. 4 has a rack member 1 having ball grooves 2 for ball screws formed on a section of the outer surface, and each end of the rack member 1 is attached to a tie-rod (not shown) connected to a knuckle for rotational supporting of the tire wheels. The ball grooves 2 are fitted with a ball screw nut 3 which is joined with a flange to a hollow motor rotation shaft 5 of a drive motor 4.
The steering system presented above is driven by powering the drive motor 4 electrically to rotate the motor rotation shaft 5, thereby rotating the ball screw nut 3 as a unit, and reciprocating the rack member 1 to left or right to change the direction of the wheels to alter the direction of travel of the automobile.
One end of the motor rotation shaft 5 (left end in FIG. 4) is freely rotatably supported by a bearing 8 firmly installed in a housing 7 which in turn is fixed to a cylindrical yoke 6. The opposite end of the motor rotation shaft 5 (right end in FIG. 4) is freely rotatably supported, by way of the ball screw nut 3, by a bearing 10 fixed in a housing 9 fixed to a yoke 6. In other words, The motor rotation shaft 5 and the ball screw nut 3 as a unit are freely rotatably supported by the yoke 6 with its associated housings 7, 9 through the bearings 8, 10. The rack member 1 and the ball screw nut 3 are disposed coaxially within the yoke 6.
A flange 3a formed on the ball screw nut 3 has a spacer 11 abutting thereto, and male threads 3b formed on the outer periphery of the ball screw nut 3 has a stopper member 12 screwed thereto. The spacer 11 and the stopper member 12 clamp the inner race of the bearing 10, and this configuration is designed to prevent the ball screw nut 3 and the motor rotation shaft 5 to displace in the axial direction even if the rack member 1 is subjected to an external axial force from the reaction of the tires against the road.
The steering system of the structure presented above requirs machining of male threads 3b on the screw nut 3 as well as components, such as the stopper member 12 to engage with the male threads 3b and the spacer 11 to clamp the bearing 10 in conjunction with the stopper member 12 to prevent the axial displacement of the motor rotation shaft 5. Therefore, not only the machining requirement of the ball screw nut 3 becomes complex, but the number of components needed for the steering system increases. Such a production process is cumbersome to manage.
Furthermore, the structure leaves some concern that the simple screw attachment of the stopper member 12 on the male threads 3b of the ball screw nut 3 may not be sufficient to prevent stretching of the stopper member 12 if the motor rotation shaft 5 should be loaded in the axial direction. One method for preventing the stopper member 12 from becoming loose is to provide a locking key. However, such an approach requires not only an additional step of machining of a seating groove extending in the axial direction but also fabrication of the locking key to engage with the seating groove and the stopper member. Therefore, this approach further increases the number of components required for the steering system. This problem is not limited to the case of attaching the ball screw nut 3 to the bearing 10, and the same problem occurs when attaching the motor rotation shaft 5 directly to the bearing 8.
To summarize the problems presented above, because the structural configuration of the driving system for power steering requires that male threads and grooves must be provided on the motor rotation shaft or ball screw nut, therefore, the manufacturing process for the steering system is not only complex and cumbersome, but demands handling of increased number of components required for the system. Further, to prevent loosening of the nut members, it is important that the components parts be manufactured with high precision, and tightening torque be carefully monitored. The ultimate result is that the conventional design for the support structure for power steering requires complex methods of preventing the axial displacement of the motor rotation shaft, and leads to high cost of manufacturing of the power steering system.